How Ramjets Work

Detonations and Arrivals

A cameraman with a high-speed camera films the thrust augmentor flame of a ramjet I-40 engine at the Lewis Flight Propulsion Laboratory in Cleveland. (The lab later became known as the John Glenn Research Center.)

Jets run on controlled explosions. That sounds strange until you realize that most car engines do, too: Pull in air, compress it, mix it with fuel, ignite it and bang! You've pushed a piston. But whereas gasoline and diesel engines involve cyclical or intermittent combustion, jets entail continuous combustion, in which fuel and air mix and burn nonstop. Either way, burning more rubber means guzzling more gas, and that means sucking in more oxygen to get the mixture right. Souped-up cars do this with superchargers; in jet engines, it's more complicated [source: Encyclopaedia Britannica].

The first operational jet aircraft zoomed into combat near the end of World War II using turbojet engines, a straightforward but ingenious design based on the Brayton (or Joule) Cycle: As the plane flies, air streams through an intake into a diffuser, a chamber that slows airflow and inhibits shock waves. It then passes through a series of bladed disks: spinning rotors, which force air backward, and stationary stators, which guide airflow. Together, they act as a compressor that pumps up pressure within the jet's combustion chambers. There, fuel mixes with pressurized air and ignites, blasting temperatures into the 1800-2800 F (980-1540 C) range or higher [sources: Encyclopaedia Britannica; Krueger; Spakovszky].

Advertisement

Pressure rises with temperature, so this explosion creates a lot of force with nothing to do but seek a quick exit. As the exhaust shoots through the rear nozzle it generates thrust to move the aircraft. En route to this nozzle, the exhaust also shoots through a turbine connected to the rotors by a torque shaft. As the turbine spins, it transfers energy to the compressor blades in front, completing the cycle.

In airplanes with turboprops or helicopters with turboshaft engines, the turbines also transfer power to a propeller or helicopter rotor via a series of gears.

Turbojets pack a lot of power but struggle at low speeds. Consequently, in the 1960s and 1970s, low-supersonic aircraft began trending toward the turbofans that most private jets and commercial airliners still use. A turbofan is the turducken of engines -- essentially a turbojet wrapped in a larger cowling with a big fan slapped on its front. The fan pulls in more air, which the engine then splits into two streams: Some air moves through the nested turbojet, while the rest flows through the empty space around it. The two streams reunite when redirected cooler air mixes with the turbojet's exhaust and slows it down, creating a larger, slower thrust stream that is more efficient at low speeds [sources: Encyclopaedia Britannica; Krueger].

Meanwhile, around the time that turbofans came into their own, research into ramjet aircraft was finally hitting its stride. It had been a long road.

Afterburners

Some turbojets and turbofans are coupled with afterburners, which eke out more energy by injecting fuel into exhaust after it passes the turbine and reigniting it. This process, also known as reheat, is inefficient but can boost turbofan thrust by as much as 50 percent [sources: Encyclopaedia Britannica; Pratt & Whitney]. Afterburners come in handy during takeoff or in unfavorable, low-speed or low-pressure conditions. They are mostly found in supersonic fighter aircraft, although the Concorde SST used them on takeoff as well [sources: Encyclopaedia Britannica; NASA; Pratt & Whitney].